.container {
  height: 130px;
}
.box_4 {
  width: 100px;
  height: 100px;
  border-radius: 20px;
  border: 1px solid black;
  animation: bbl_radius 5s infinite;
}
@keyframes bbl_radius {
  50% {
    border-bottom-left-radius: 50px;
  }
}